RIO DE JANEIRO – A fire of as yet unknown size destroyed part of the roof of the Olympic velodrome in Rio de Janeiro, the structure used least since the 2016 Games and the construction of which was delayed longest and one of the costliest projects undertaken by the city.

The fire at the cycling track began about 1:00 am on Sunday, just six days away from the one-year anniversary of the Rio Olympics’ inaugural ceremony, and authorities attributed it to a paper balloon with a burner inside that landed on its roof, although such objects are prohibited in Brazil.

Firefighters said they needed at least two hours to bring the blaze under control, but about 8:00 am they were still working to put out the embers and remove the ruins of the structure located in the Barra de Tijuca district in western Rio.

Sports Minister Leonardo Picciani lamented the tragedy in a communique, but refrained from discussing the roof damage, and it was unknown whether the track, the seating or the building itself were damaged.

He said that authorities would await the results of the investigation to determine who bears responsibility for the fire, and they will evaluate the damage and the measures to be taken to repair “this important national property.”

The velodrome, one of the structures comprising the Olympic Park and said to have one of the fastest cycling tracks in the world, was used for cycling competitions during the Olympics last August but since then it has remained practically unused.

The cycling track cost 143 million reais (about $45 million), was inaugurated in June 2016, six months behind schedule, and was the last Olympic project to be completed.

Despite the lack of use the venue has had since the Olympics, it costs about 11 million reais per year to maintain, since its special and delicate track requires constant use of air conditioning.

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